1. Field of the Invention
The present invention relates to a Gigabit-capable passive optical network (hereinafter, referred to as GPON). More particularly, the present invention relates to a multicast transmission method using GEM (GPON Encapsulation Method) Mode.
2. Description of the Related Art
In order to construct a subscriber network from a telephone office to a building or a general house, there are various network structures and progress schemes that have been recently proposed. For instance, an x-digital subscriber line (XDSL), a hybrid fiber coax (HFC), a fiber to the building (FTTB), a fiber to the curb (FTTC), or a fiber to the home (FTTH) all have been proposed in recent years. From among these structures, an FTTx (x represents B, C, or H) structure may be classified into an active FTTx structure that is realized by construction of an active optical network (hereinafter, referred to as AON), and a passive FTTx structure which is realized by construction of a passive optical network (hereinafter, referred to as a PON).
Herein, since a PON, which is employed for realizing a passive FTTx, has a point-to-multipoint topology by passive elements, it is proposed that a PON be selected as a scheme of choice for realizing an economic subscriber network. In other words, in the PON, one optical line termination (hereinafter, referred to as an OLT) is connected to a plurality of optical network units (hereinafter, referred to as ONUs) by means of an optical distribution network (hereinafter, referred to as an ODN) of 1×N. This connection to a plurality of optical networks utilizes a distributed topology with a tree structure that is formed.
FIG. 1 is a block diagram of a conventional PON. Generally, a PON includes one OLT and a plurality of ONUs. FIG. 1 shows an example in which one OLT 10 is connected to three ONUs 12a to 12c through an optical distribution network ODN 16.
A first type of PON developed and standardized was an a synchronous transfer mode passive optical network (hereinafter, referred to as an ATM-PON), and the standardization contents are written in the ITU-T G.982, ITU-T G.983.1 and ITU-T G.983.3 series that are drawn up by the international telecommunication union's telecommunication section (ITU-T).
FIG. 2 illustrates a protocol stack structure of a conventional GPON. Referring to FIG. 2, the protocol stack of the GPON includes a protocol layer 100 which interfaces with an upper layer, a GTC layer 200, and a GPON physical media dependent (GPM) layer 300. The protocol layer 100 comprises an ATM client 110, an ONT management control interface (hereinafter, referred to as an OMCI) 120, a GEM client 130, and a physical layer operation administration maintenance (PLOAM) module 140.
The OMCI has been drafted into the Ggpon.omci and its contents have been published, which is a protocol for controlling an ONU. The OMCI is classified into an ATM mode OMCI and a GEM mode OMCI. The ATM mode OMCI is based on definition of G.983.2 and the GEM mode OMCI is proposed for utilization of a GEM frame header and a payload structure used in an ATM mode. Herein, the main contents of the OMCI message includes a service setting and an assignment of a virtual path identifier (VPI)/a virtual channel identifier (VCI), or a setting for a data path assigning port IDs, etc.
In the GPON protocol having this structure, the GTC layer 200 converts upper frames into a GTC frame and subsequently transmits the frame. From among the upper frames, the ATM client 110 supports a transmission method of an ATM mode, and the GEM client 130 supports a transmission method of an GEM mode.
The ATM client 110 fits the transmission data to the GTC frame by the unit of a cell having a fixed length. In this case, if there is a portion that has a length shorter than the length of a cell (typically, 53 bytes) in the GTC frame, the ATM client 110 maps transmission data to the next frame and then transmits the frame. Accordingly, cells are not divided in the ATM mode.
However, since a GEM frame is a packet having a varied length, when the GEM client 130 shows that the packet fits the GEM frame to the GTC frame, the GEM client 130 may divide the GEM frame and then transmit the divided portions of the frame, to increase the efficient use of available bandwidths.
For instance, when the GEM client 130 receives user data from an upper layer, the GEM client 130 receives information (e.g. length) about the GTC frame, which is standing by, from the GTC layer 200, and divides the user data on the basis of the information (e.g. length), and generates a plurality of GEM frames. Otherwise, the GEM client 130 does not divide the user data, generates only one GEM frame, and then transmits the GEM frame to the GTC layer 200. Then, the GTC layer 200 fits the GEM frame to a GTC frame that is standing by, and transmits the frame. Also, a reception side reassembles the divided GEM frame and then transmits the frame to an upper layer.
In addition, a data transmission method in a network includes a unicast transmission method for one-to-one transmission; a broadcast transmission method for transmission of one-to-all nodes; a multicast transmission method for transmission of one-to-predetermined multiple nodes for multiple subscribers. Generally, the unicast transmission method and the broadcast transmission method are basically provided. However, in the case of the multicast transmission method, protocols for the multicast transmission and support in a corresponding network are necessary.
FIG. 3 shows a process of transmitting multicast frames in an GEM mode in an existing (a current) conventional GPON. Referring to FIG. 3, it is first assumed that members of a multicast group M registered in a router 31 are hosts a, b and c. When a multicast frame generated from the construction of FIG. 3 is transmitted to each multicast member host through a GEM mode of the GPON (which does not support a multicast), the GPON operates as follows:
For an Ethernet service in the GPON, in the GEM mode, one or more ports are assigned between an OLT 32 and an ONTs (or ONUs), and the ports are differentiated from each other by port IDs, such as P1, P2 and P3 shown in the FIG. 3. The OLT 32 must determine ports through which a received multicast frame will be transmitted. From the result of the determination, port IDs P1, P2, P3 are recognized as registered ports registered of a multicast group. As a result, the multicast frame is transmitted through the recognized ports.
Specifically, a multicast frame 301 transmitted from the router 31 is transmitted to the OLT 32 with respect to ONTs which have been already registered to the router 31, which supports a multicast transmission method, as a members of a multicast group by means of an IGMP packet. Furthermore, the OLT 32 generates a multicast frame according to each of the ONTs by means of port IDs of the ONTs registered as the members of the multicast group, and then transmits the frame.
Herein, an operation of the OLT 32 will be described with reference to FIG. 3. The OLT 32 transmits a received multicast frame to ONTs 33-1 to 33-3, which are registered as the members of the multicast group, by means of port IDs P1, P2, P3 of each ONT. That is, the OLT 32 adds the received multicast frame to each of port IDs 302 to 304 corresponding to the ONTs 33-1 to 33-3, transmits the multicast frames, and each of the ONTs receives a packet corresponding to its own port ID. Accordingly, the OLT can perform corresponding multicast transmission with respect to the ONTs registered as the members of the multicast group.
As described above, the existing multicast transmission method according to a GEM mode of a GPON does not make the best use of the advantages realized by multicast transmission, and it has been processed by a way similar to a gathering of a plurality of unicast transmission.
As a result, the number of frames actually transmitted through the GPON is three times, as shown in FIG. 3 (this results from an example in FIG. 3, and it is apparent that the number of frames varies according to the number of ONTs belonging to the multicast group). This causes the same frame to be transmitted three times, and a majority of bandwidth is thus wasted.
As described above, since a multicast transmission method is not supported in the GEM mode of the conventional GPON, a multicast frame must be copied as many as the number of members of the multicast group and transmitted, and bandwidth is thus wasted.